Internal loading cylindrical filters are well known, and U.S. Pat. No. 4,154,588 is exemplary of current developments in the art. This patent discloses a knit pile fabric filter element which is stretched in tubular form over a helically-wound metal rod support frame. The pile of the fabric is on the inside, and a lengthwise zipper is provided in the tubular fabric filter element for ease of assembly over the helical support. The small helically-travelling cleaning nozzle of the filter apparatus has been found not to be entirely suitable in applications where large quantities of fine wood and paper fiber are being recaptured for reuse after the fiber has been concentrated in smaller amounts of air by passage through a cyclone separator.
U.S. Pat. Nos. 1,944,267 and 1,944,268 disclose the only known unsupported cylindrical fabric filter elements with self-cleaning nozzles; however, in these patents the nozzles are of annular configuration and move lengthwise through the cylindrical filter elements, piston-like, and the axes of the cylindrical filter elements are in a vertical position for a "bag-house" application whereby it might be assumed that the long, narrow, heavy, woven, woolen tubular filter elements used for filtering purposes at that time would be relatively rigid, and that the piston-like movement of the cleaning nozzles therefore had no tendency to disturb the original cylindrical form of the filter element, but rather to maintain it. Moreover, the vertical disposition of the filter elements did not result in any tendency of such elements to bow at the center if not supported, as would be the case if the filter elements were arranged horizontally between two end plates.
Two additional U.S. Pat. Nos. 1,995,648 and 2,057,446, having the same inventor as the last-mentioned pair of patents, disclose vertical cylindrical filter elements in which the filter medium is a fibrous material such as steel or mineral wool, and the medium is fastened to a supporting screen or wire mesh cover for greater strength, rigidity, and durability of the filter element. In both of these patents, the cylindrical proportions of the filter elements are those of conventional drum filters, unlike bag house filter elements, and full length rotating suction nozzles serve to clean the filtered-out waste material from the interior of the filter element, with biasing means being provided to assure that the filter elements and suction nozzles stay in contact.
The present invention provides the advantage of a relatively inexpensive, very flexible knit fabric filter medium having a pile component which provides a "surface loading" filter effect wherein the pile fibers lie down as the dirty air passes through the filter element from the pile side, thereby trapping the waste dust or fibers on the surface; then, as the suction nozzle passes, the pile fibers are sucked up perpendicular to the filter element and the waste material is easily and thoroughly sucked therefrom. The knit filter element in tubular form is quite flexible and can be readily stretched taut between the end plates of the filter apparatus to assume a cylindrical shape in which the internal rotating cleaning nozzles are adjacent the knit fabric, and the filter element may be assembled in place from the ends of the filter apparatus without need for access from the sides thereof (which are usually inaccessible due to the typical "built-in" arrangement of the filter apparatus), with a unique end support structure being provided for the filter element to permit this ease of assembly. In some cases of extremely heavy waste build-up inside the filter element, the force of air passing through the knit fabric element causes it to have a tendency to "balloon" out of its normal taut cylindrical shape, and, in accordance with a further feature of the present invention, restraining circumferential flexible strands may be used to restrain the filter element from such ballooning tendencies so that the rotating nozzle may clean it suitably.